Sustainability Indicators

Sustainability Indicators are measurements of groundwater-related characteristics obtained from data collection efforts. Indicators provide evidence about the condition of the groundwater system and (or) how it has changed relative to sustainable groundwater use, which is groundwater use balanced to meet societal, economic, and environmental groundwater needs. Indicators are thus the core measurements used to evaluate groundwater sustainability. To review the USGS study where these indicators were first published as well as other reports see the Upper San Pedro Partnership’s Studies and Reports.

The Upper San Pedro Partnership (USPP) has grouped the sustainability indicators into four main categories: Regional Groundwater Indicators, Near-stream Groundwater Indicators, River Flow Indicators, and Springs Indicators.

Regional Groundwater Indicators

The Regional Groundwater Indicators group provides evidence about the condition of the ecosystem dependent upon the regional aquifer and enables tracking of how regional groundwater levels have changed or are trending over time. Also referred to as “subwatershed-wide indicators”, these enable evaluation at a whole basin scale and include indicators 1 through 4: regional-aquifer water levels, regional-aquifer horizontal gradients, groundwater storage change measured with microgravity, and groundwater budget balance.

Indicator 1. Regional-aquifer water levels

Groundwater levels below land surface measured in non-pumping wells across the Sierra Vista Subwatershed that are screened in the regional aquifer.

When many regional-aquifer water level measurements are available through time, they can provide a broad overview of changes occurring in the aquifer due to pumping, changes in climate, changes in land use, or other factors. Water levels can steadily increase or decrease (depth to water below land surface steadily decreases or increases) or vary (depth to water below land surface increases and decreases but not steadily in one direction). Water levels rarely remain static, neither increasing nor decreasing; more commonly that indicates a plugged well and the water levels are no longer connected to the aquifer.

Indicator 2. Horizontal hydraulic gradients (regional-aquifer wells)

The difference in groundwater elevation (relative to sea level) as a function of distance from one regional-aquifer well to another along a transect of 2 or more wells.

The change in horizontal gradient can be calculated between wells that are at different distances from an area of groundwater discharge (for example, a stream channel). A negative value in the horizontal gradient indicates that the water-level elevation is lower at well A, near the pumping center, than it is at a well B, closer to the basin center (the area of groundwater discharge). A positive value indicates the opposite. Areas with negative horizontal-gradient values would be interpreted as flowing toward the pumping center, rather than toward the riparian area and river. Areas with declining (but not necessarily negative) horizontal gradients would be interpreted as seeing a reduction in the tendency of groundwater to flow toward the river in addition to that resulting from general reductions in groundwater levels (indicating a reduced volume of water flowing toward the river). Areas of increasing horizontal gradients would be interpreted as seeing an additional increase in the tendency of groundwater to flow toward the river.

Indicator 3. Aquifer storage change measured with microgravity

A way to directly determine changes in aquifer storage by using a gravity meter to measure the changes in the acceleration due to gravity brought on by changes in the mass—resulting from changes in the water stored—in the aquifer. Microgravity can also be used to estimate the difficult-to-determine aquifer property specific yield.

When water increases or decreases in an aquifer, there is a change in mass. When there is greater mass underground (i.e., there is more water), the acceleration due to gravity is greater than when there is less mass (i.e., there is less water). The difference in gravitational acceleration that results from changes in the amount of water present in an aquifer can be measured using a gravity meter, and the change in gravity can be converted to the change of water present in the aquifer (the aquifer storage).

Small changes in the acceleration due to gravity, measured at the land surface, are directly related to the total change in water stored in an aquifer, expressed as the height of a slab of free-standing water. When these changes in storage appear to be related to nearby changes in water levels in wells, we obtain information about how much water is gained or lost from aquifer storage for a given increase or decrease in water level (specific yield). It is also possible to estimate specific yield when gravity-change observations are made at a well where concurrent water-level measurements are made, and gravity and water-level changes are occurring consistently over a period of time.

Indicator 4. Groundwater-budget balance

An accounting exercise, not a direct measurement, that is calculated by summing all the different kinds of recharge to the groundwater system, summing all of the different kinds of discharge from the groundwater system, and then subtracting the total discharge from the total recharge to the regional aquifer. The result is a crude, single-value estimate of the surplus or deficit in the groundwater supply across an entire watershed. The resulting balance indicates whether the regional aquifer is losing or accreting storage. There is uncertainty on the order of 1,000s of acre-feet in the value of the Sierra Vista Subwatershed water budget balance.

The groundwater-budget provides a crude, single-value estimate of changes to the groundwater supply across the entire Sierra Vista Subwatershed. When the groundwater-budget balance remains greater than 0 acre-ft, it suggests that more groundwater will move toward the San Pedro River. When the groundwater-budget balance remains less than 0 acre-ft, it suggests the opposite—that less groundwater will move toward the river. At what point in time a change in the groundwater-budget indicator might affect the San Pedro River, however, is dependent on where the primary impacts (groundwater recharge or withdrawal) are taking place. In general, it is assumed that the closer groundwater recharge or discharge is to the river, the sooner these effects will affect river base flow and riparian health.

Near-stream Groundwater Indicators

The Near-stream Groundwater Indicators group provides evidence about the condition of the ecosystem right along the San Pedro River and enables tracking of how it has changed or is trending over time. Also referred to as “riparian system indicators,” indicators 5 through 7 are focused on the alluvial aquifer adjacent to the San Pedro River, and include: near-stream alluvial-aquifer water levels, near-stream vertical gradients, and annual fluctuation of near-stream alluvial-aquifer water levels.

Indicator 5. Near-stream alluvial-aquifer water levels

Groundwater level below land surface measured in non-pumping wells across the Sierra Vista subwatershed that are screened in the near-stream alluvial aquifer.

Alluvial groundwater (or bank storage) can come from flood flows and enhanced-infiltration projects, as well as from a regional aquifer. Regardless of the source, increases in alluvial-groundwater levels are a direct and immediate indication of an increase in groundwater available to riparian plants and to discharge to the San Pedro River; decreases in alluvial-groundwater levels conversely indicate a loss of available groundwater for plants and the river. The lowest dry-season water levels are well correlated with riparian-system health. Along the San Pedro River, a multi-year declining trend in near-stream alluvial-aquifer levels may imply a decrease in groundwater that is available to the riparian area and as base flow to the stream. An increase in water levels may imply the reverse, an increase in groundwater supply available to the riparian area and the stream.

Indicator 6. Near-stream vertical gradients

The difference in groundwater elevation (relative to sea level) as a function of difference in height of the screens (opening to groundwater) in two different wells that are typically located close to each other.

Near-stream vertical gradient values show whether groundwater is moving toward the stream channel (upward gradient; gaining reach) or from the stream channel toward the deeper regional aquifer (downward gradient; losing reach). The vertical gradient is calculated by taking the water-level elevation in the deep well minus the water-level elevation in the shallow well, and dividing it by the difference in the depth of the screen in the deep well minus the depth of the screen in the shallow well. The greater the magnitude of this value, the stronger the gradient.

Indicator 7. Annual fluctuation of near-stream alluvial-aquifer water levels

The annual range of groundwater depths measured in continuously monitored, near-stream alluvial-aquifer wells.

This indicator is related to the health of the riparian forest—the less water levels fluctuate the healthier the trees. It is best evaluated through a continuous measurement of water levels in near-stream wells, ideally with pressure transducers recording water-level depth at least every 6 hours. The highest annual water level is selected from “winter” months, November to April, and annual dry-season low-water level is selected from summer months, May through September. Typically, the highest annual winter water levels occur in March, but highest winter water levels have been known to occur in all months from November to April. Typically, the lowest summer dry-season water levels occurred in June or July, but in years of less summer rain, such as 2009, the low did not occur until September at some locations.

River Flow Indicators

The River Flow Indicators group provides evidence about the condition of the riverine ecosystem and enables tracking of how river flows have changed or are trending over time. Also referred to as “San Pedro River indicators”, these are related to the river itself, and include indicators 8 through 12: streamflow permanence, base-flow on San Pedro and Babocomari Rivers, June wet-dry status, San Pedro River water quality, and San Pedro and Babocomari Rivers stable isotopes.

Indicator 8. Streamflow permanence

The percent of a year water is present at a given location in the SPRNCA  measured by evaluating data from 12 locations using 8 photography sites, 1 stage recorder site, and 3 gaging station sites.

Streamflow permanence is measured by automatic remote digital cameras at 8 locations and by USGS stream gaging stations (including the Lewis Springs Stage Recorder) at 4 additional locations. From 2000 to 2003, a more comprehensive method of estimating streamflow permanence throughout the SPRNCA was employed that used temperature data loggers. Streamflow permanence explains more of the variance in the size (basal area) of cottonwoods and willows than either groundwater depth or groundwater fluctuation. Sites used for streamflow permanence are not necessarily representative of the entire stream reach, or even conditions nearby as there is great variability in flow conditions from place to place along the San Pedro River, especially during the dry late spring and early summer months.

Indicator 9. Base flow discharge on San Pedro and Babocomari Rivers

The amount of groundwater discharge that flows in a stream measured at USGS stream gages.

Base flow refers to water that flows in the San Pedro River and Babocomari River channels in the absence of any immediate influence from storm runoff, and is composed of regional groundwater, alluvial groundwater, or both. Sometimes only discharge from a regional aquifer is counted as base flow; however, it is prohibitively difficult if not impossible to differentiate contributions of near-stream alluvial (bank storage) discharge from regional aquifer discharge in the case of the San Pedro River, and so all water present during low flow conditions is considered base flow. Base flow can be computed a few different ways. For the WHIP, the January 3-day low flow, which is largely independent of ET, is used to represent base-flow conditions. Three of the 4 discharge gaging stations where base flow is estimated are typically dry in June, the driest time of the year, so to estimate annual base flow on the San Pedro, the lowest average value for 3 consecutive January days is used, when base flow is typically present at all sites. Because the Charleston gaging station is in a perennial location, June 3-day low flow can be calculated for this one gaging station and so is also included.

Indicator 10. June wet-dry status

A mainly 1-day, on-the-ground visual evaluation of the San Pedro River during the driest time of the year to determine location and length of wetted stream lengths as well as total wetted length of the river measured with GPS units by agency staff and citizen scientists.

The wet-dry mapping fieldwork is conducted by volunteer citizen scientists who survey assigned river reaches on foot or horseback. Using hand-held GPS units, volunteers record the start and end points of each wetted reach. A “wetted reach” has to be at least 30 ft in length and separated from adjacent wetted reaches by at least 30 ft of dry streambed. Two or more wetted sections separated by less than 30 ft of dry streambed are considered a single wetted reach. The resulting maps, created in GIS, determine where a spatially intermittent stream has surface flow and where it is dry at a specific time of the year.

Indicator 11. San Pedro River water quality

Surface-water samples collected near the Charleston stream gaging site from 1964 to 2012 and analyzed for a suite of water-quality analytes and parameters.

A wide variety of water-quality data have been obtained from San Pedro River surface-water samples collected at the Charleston stream-gaging location since 1964, and the location has been a long-term National Water-Quality Assessment (NAWQA) reference site from 1991 to 2012. During this later period, all samples were collected with nationally consistent protocols. To better understand the trends in the water-quality analytes of interest, the analytes were analyzed on the basis of four components—their relation to (1) discharge, (2) seasonality, (3) long-term trend, and (4) a random component using Hirsch and Di Cicco’s (2014) Exploration and Graphics for River Trends (EGRET-WRTDS) tool. Most data were collected from 1987 to 2015. Sampling included field parameters, major-ion concentrations, nutrients, suspended sediment, and trace elements.

Indicator 12. San Pedro and Babocomari Rivers isotope analysis

Concentrations in water of stable isotopes of oxygen and hydrogen can provide information about the source of groundwater including its elevation of recharge and also about any changes in the relative contribution of groundwater to surface streams.

Analysis of stable-isotope ratios of oxygen (δ18O) and hydrogen (δ2H) is used to investigate groundwater inputs to river flow and evaluate how that may be changing through time. An increasing trend in the concentration of δ18O in surface water at a given location could reflect a decrease in the amount of groundwater entering the stream; conversely, a decreasing trend in δ18O could reflect an increase in the amount of groundwater entering the stream. The concentration of δ18O tends to be higher when evaporation occurs, and this implies that the groundwater contribution to the stream (no evaporation occurring) relative to that from upstream surface water (evaporation occurring) is decreasing.

Springs Indicators

The Springs Indicators group provides evidence about the condition of available water for springs and enables tracking of how springs are trending over time. This category of indicators is based on characteristics of springs in SPRNCA, and include indicators 13 and 14: springs discharge and springs water quality.

Indicator 13. Springs discharge

The amount of water discharging from a spring source per unit of time measured quarterly at 4 springs and 1 flowing well using a manual method such as a current meter, a flume, or a container of known volume.

The Sierra Vista subwatershed springs include 3 springs on the west side of the river and one on the east side, plus one flowing (also called artesian) well on the east side (the McDowell-Craig Farm well). A spring is a point of groundwater discharge to the surface, and thus spring discharge measurements provide information about trends in groundwater flow from areas of natural or managed recharge. The greater the discharge at a spring, the greater the recharge that must be occurring upgradient of the spring, and vice versa.

Indicator 14. Springs water quality

Surface-water samples from spring flow were analyzed for wastewater and pharmaceutical suites four times spanning 2006 to 2010, collected with passive samplers at Murray, Horsethief, and Lewis Springs.

Murray Spring was sampled 4 times (2006, 2008, 2009, 2010) and Horsethief and Lewis Spring were each sampled once (2009). Water quality analyses included wastewater and pharmaceutical suites as well as field parameters. Passive samplers can detect compounds at lower concentrations than discrete sampling (volumes of water collected at a single location at one point in time). In 2009, passive samplers were deployed at Murray, Horsethief, and Lewis Springs. In 2006 and 2010, only discrete samples were acquired from Murray Springs, and in both 2009 and 2010 discrete samples were also acquired from the Environmental Operations Park (Sierra Vista’s wastewater treatment facility).

Other Non-indicator data types

Non-indicator and climatic reference data included in the WHIP  are helpful to look at alongside the Sustainability Indicators, and include precipitation, groundwater pumping, population estimates.

Non-indicator data type - Precipitation

The depth of liquid water that falls as rain, snow, hail, sleet, etc. across a particular area in a particular amount of time.

Two kinds of precipitation data are available on the WHIP:

  • Annual precipitation data from a network of precipitation gages that are maintained by the Agricultural Research Service
  • Arizona climate division 7 annual precipitation data which are based on National Weather Service Cooperative Observer Program (COOP) data with various adjustments made for station changes, biases, missing data, spatial interpolation and the like (https://www.ncei.noaa.gov/products/land-based-station/cooperative-observer-network). While not solely for the areal extent covered by the WHIP, these data provide a reasonable approximation of the long-term precipitation trends for the Upper San Pedro Basin.

Non-indicator data type- Population

The total number of people in the Upper San Pedro basin, by Subwatershed.

Population from US Census data every 10 years with intervening years obtained from the State of Arizona data estimates. Accurate counting along subwatershed boundaries requires obtaining data at the census block level although reasonable approximations can be made on a percent areal basis of census tracts that fall both in- and outside of the Upper San Pedro Basin if census block data are not available.

Non-indicator data type- Groundwater Pumping

Water pumped out of the ground for any reason and by anyone.

There are 6 groundwater pumping categories: 1. Water Company (mainly domestic supply); 2. Exempt Well (as defined by Arizona Department of Water Resources as wells with a pumping capacity of 35 gallons per day or less); 3. Turf Irrigation (including parks and golf courses); 4. Sand and Gravel Operations (based on any increase in population); 5. Stock Tank (estimated based on animal units in the Sierra Vista Subwatershed); 6. Agricultural Irrigation (estimated by USGS Water Use program).

Non-indicator data type - EOP (Environmental Operations Park) Recharge

The total volume of treated effluent recharged annually through a number of specially designed and maintained recharge basins 3 miles west of the San Pedro River. A smaller amount of project water enters the subsurface through the on-site wetlands.

The City of Sierra Vista's Environmental Operations Park (EOP) is the largest wastewater treatment facility in the Sierra Vista Subwatershed and also its largest managed recharge project. Constructed in the early 2000s through an agreement with the U.S. Bureau of Reclamation, the project began processing and recharging Sierra Vista wastewater in July, 2002. The EOP treatment process was further updated in 2012 with the construction of the Clarifier Retrofit Project.

Non-indicator data type - Stream Discharge Data

Discharge is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic feet per second. "Stage" is the height of the stream above a fixed location, typically close to the stream bed. 

Discharge data are currently available from the USGS NWIS website for 10 active streamflow gaging station locations, 1 historic gaging station location (Greenbush Draw), and 3 stage recorders (no discharge). Discharge data are currently available from the USGS NWIS website for all 14 locations via a web link.